A common form of dockboard is installed in a shallow depression or pit in a loading dock and includes a ramp that is hinged at its rear edge to the dock and an extension lip is hinged to the front edge of the ramp. The ramp is adapted to be moved from a generally horizontal cross traffic position, where it is flush with the upper surface of the dock, to an upper inclined position. Due to the substantial weight of the ramp and lip, a counterbalancing mechanism, such as a spring, is employed to bias the ramp upwardly to the inclined position. In addition, a releasable holddown mechanism interconnects the dock and the ramp and locks the ramp against upward movement unless manually released.
When not in use, the ramp is locked or held down by the holddown mechanism in the cross traffic position. When a truck or other carrier backs into position in front of the dock, an operator manually releases the holddown mechanism so that the counterbalancing spring will then pivot the ramp upwardly to the inclined position. As a consequence of the upward pivotal movement of the ramp, a lip lifting and latching mechanism is actuated to pivot the lip outwardly to an extended position. The operator then walks outwardly on the inclined ramp and his weight will overbalance the force of the counterbalancing spring to lower the ramp and move the extended lip into engagement with the bed of the truck.
The conventional holddown mechanism is a one-way locking device which is arranged so that the ramp can be moved downwardly without restraint yet the holddown mechanism, unless manually released, will prevent upward movement of the ramp.
U.S. Pat. No. 3,646,627 describes a holddown mechanism for a dockboard which incorporates a pawl and ratchet mechanism. The holddown mechanism of the aforementioned patent includes a tubular housing having one end pivotally connected to the frame and a compression spring is mounted with the housing. A tube is positioned concentrically within the spring and the force of the spring acts against the cap on the end of the tube to urge the tube into a telescoped position with respect to the housing.
Mounted for sliding movement within the tube is a ratchet bar and the outer end of the ratchet bar is pivotally connected to the ramp. The ratchet bar is locked with respect to the tube by a double pawl mechanism which engages the teeth of the ratchet. To release the pawls from the ratchet bar, an operator pivots a handle through an operating cable which acts to pivot the pawls out of engagement with the ratchet bar teeth and thereby enables the counterbalancing mechanism to elevate the ramp to the inclined position.
When the ramp is walked downwardly by the operator, the ratchet bar will move inwardly with respect to the tube and the pawls and ratchet bar are arranged so that the ratchet bar can move to the retracted position without locking interference from the pawls.